An electronic ballast intended to operate high pressure arc lamps of the metal halide variety preferably comprises an automatic control system designed to maintain constant arc watts over a range of primary supply voltages and arc operating voltages. One prior art approach is to use a multiplier as part of the constant watts control system. In such case, arc voltage and arc current feedback signals are suitably scaled and multiplied together. The multiplier output is then subtracted from a reference signal. The difference signal resulting from this subtraction is averaged, amplified and applied to a power controller that adjusts the arc current. The automatic control system then regulates the product of arc volts and arc amperes (arc watts) resulting in an ideal (i.e., constant) arc watts-versus-arc volts characteristic.
However, multipliers are preferably not utilized to combine the voltage and current feedback signals in an electronic ballast. Instead, in another prior art approach, the (suitably scaled) signals are preferably simply summed together, that sum being held constant by an automatic control system. This is because the summing technique yields more constant watts, usually without trimming, and with lower cost than use of a multiplier over a typical arc operating voltage range. Trimming means manual adjustment of circuit values in production, entailing the use of expensive, skilled labor.
The prior art summing technique, however, may require trimming if the operating arc voltage range is wider than usual in combination with a narrow watts specification. It, therefore, would be desirable to provide a constant arc wattage ballast for a lamp that eliminates the need for circuit trimming.